Packet transmission system, packet transmission method, packet transmission program, and recording medium on which the recorded program is recorded

ABSTRACT

An OLT  1  sequentially allocates transmission permission of a packet on an upstream transmission line to a plurality of ONUs  2  registered, and performs packet communication in a simplified method with control by using a first timer for detecting a packet signal and a second timer for controlling transmission permission time, in order to transmit an IP packet of variable length without mounting a function of complex processing for analyzing an upstream packet in the OLT  1.

TECHNICAL FIELD

[0001] This invention relates to a packet communication system and itsmethod for communicating IP (INTERNET PROTOCOL) packet data of variablelength to a user network interface (UNI: USER NETWORK INTERFACE).

BACKGROUND ART

[0002] As an example of related art, FIG. 12 shows a block chart forexplaining a system configuration of an optical communication apparatusand its communication method and a recording medium in which its controlmethod is described in Japanese Unexamined Published Patent PublicationHEI 11-98151 for example.

[0003] In FIG. 12, the optical communication apparatus includes astation equipment 111 and a subscriber's equipment 113. An externalworkstation 112 is connected to the station equipment 111, and aterminal 114 is connected to the subscriber's equipment 113.

[0004] The station equipment 111 includes a function of allowingchanging bands dynamically. In a PON system, an excess band between thesubscriber's equipment 113 and the station equipment 111 excluding bandassurance service is shared among a plurality of subscribers subscribingto band non-assurance type service by using the function of the stationequipment 111.

[0005] The subscriber's equipment 113 includes a PDS (Passive DoubleStar) terminating unit 131 for terminating an interface with the stationequipment 111 facing to the subscriber's equipment 113, a terminalinterface terminating unit 133 for terminating the band non-assurancetype service in a user side, a memory 132 for performing formatconversion between the PDS terminating unit 131 and the terminalinterface terminating unit 133 and storing packet data, and a memorycontrol unit 134 for measuring data accumulation volume in the memory132 and notifying the PDS terminating unit 131 of transmission requestwhen a signal is needed to be sent to the station equipment 111 side byopening the PDS terminating unit 131.

[0006] The station equipment 111 includes a PDS terminating unit 121 forterminating an interface with the subscriber's equipment 113 facing tothe station equipment 111, a control unit 125 for terminating controlinformation from the external workstation 112 and distributing necessarycontrol information to each of function blocks in the equipment, asubscriber's information accumulating unit 124 for accumulatinginformation on a subscriber who has applied to use a shard band in thecontrol information from the control unit 125, a packet analyzing unit123 for receiving subscriber's information from the subscriber'sinformation accumulating unit 124 and sending a subscriber's ID and apolling instruction to the PDS terminating unit 121 and a time-divisionswitch (TSW: TIME DIVISION SWITCH) 122.

[0007] When the packet analyzing unit 123 has transmitted thesubscriber's ID and the polling instruction to the PDS terminating unit121, the packet analyzing unit 123 receives transmission request 151 anda subscriber's ID from the relevant subscriber's station 113. When thepacket analyzing unit 123 has received the transmission request, thepacket analyzing unit 123 sends the relevant subscriber's equipment 113a transmission permission signal 152 for upstream shared queuing whichis shared by using the above-stated function, and receives a packetsignal from the subscriber's equipment 113. Further, the packetanalyzing unit 123 analyzes a packet length signal described in a packetoverhead (LLC part) in a first packet signal from the subscriber'sequipment 113, determines time of occupancy by the relevant subscriber'sequipment based on packet length, sends the transmission permissionsignal 152 to the subscriber's equipment 113 only for the time ofoccupancy, and receives the packet signal from the subscriber'sequipment 113.

[0008] As stated, the above-stated related art has adopted a method thatwhen an OLT (OPTICAL LINE TERMINATION) as the station equipment 111allocates a transmission band for an upstream direction to ONUs (OPTICALNETWORK UNIT) which are a plurality of subscriber's equipments 113, theOLT sends subscriber's ID information and polling instructioninformation to an arbitrary ONU and receives transmission requestinformation and the subscriber's ID information from the relevantsubscriber's equipment 113, and further when the OLT has received thetransmission request information, the OLT sends a transmissionpermission signal to the relevant subscriber's equipment 113 andreceives the packet signal from the relevant subscriber's equipment 113.Further, the packet analyzing unit in the OLT has adopted a method ofanalyzing the packet length signal described in the packet overhead (LLCpart) in the first packet signal from the subscriber's equipment 113,determining the time of occupancy by the relevant subscriber's equipment113 based on the packet length, sending the transmission permissionsignal to the subscriber's equipment 113 only for the time of occupancy,and receiving the packet signal from the relevant subscriber's equipment113. Therefore, it has been necessary that the OLT has complex functionof controlling processing and function of analyzing a packet.

[0009] This invention aims at performing packet communication in asimplified sending and receiving method by sequentially allocatingtransmission permission of a packet on an upstream transmission line bythe OLT to the plurality of ONUs registered.

DISCLOSURE OF THE INVENTION

[0010] For achieving the above-stated object, a packet communicationsystem according to this invention includes a master station fortransmitting a downstream packet signal including identificationinformation for identifying transmission permission in order to givetransmission permission without receiving transmission request to themaster station itself, and a slave station for receiving the downstreampacket signal transmitted from the above master station and judging iftransmission has been permitted based on the identification informationin the downstream packet signal received.

[0011] Further, the above slave station transmits an upstream packetsignal to the master station when the slave station judges thattransmission has been permitted.

[0012] Further, the above master station includes a timer 1 fordetecting packet which measures limit time when the master station canjudge if the upstream packet signal has been transmitted from the aboveslave station to which transmission has been permitted, and the masterstation judges if the master station has received the upstream packetsignal transmitted from the above slave station within the limit timemeasured by the timer 1 for detecting packet, stops the timer 1 fordetecting packet when the master station has received the upstreampacket signal, and updates the identification information foridentifying transmission permission when the master station has notreceived the upstream packet signal.

[0013] Further, the above master station includes a timer 2 forpermitting transmission which measures limit time when the masterstation can continuously receive the upstream packet signal transmittedfrom the above slave station to which transmission has been permitted,and the master station judges if receipt of the upstream packet signaltransmitted from the above slave station has ended within the limit timemeasured by the timer 2 for permitting transmission, stops the timer 2for permitting transmission when the receipt has ended, and updates theidentification information for identifying transmission permission whenthe receipt has not ended.

[0014] Further, the above master station transmits a downstream packetsignal including overhead information indicating time when output of theupstream packet signal is unstable, and the above slave station adds anoverhead which is a group of signals which are not reproduced by theabove master station on a head of the upstream packet signalcorresponding to the overhead information in the downstream packetsignal transmitted from the above master station.

[0015] Further, the above master station includes a buffer foraccumulating the downstream packet signal separately according to signaltransmission speeds, and the master station reads out the downstreampacket signal of various speeds accumulated in the buffer separatelyaccording to the signal transmission speeds, transmits the downstreampacket signal of the various speeds read out, and permits a slavestation which has been able to be synchronized with transmission speedof the downstream packet signal to transmit an upstream packet signal.

[0016] Further, the above master station monitors upstream packetcommunication volume of each slave station and controls a value of thetimer 2 for permitting transmission for the above slave station based onthe upstream packet communication volume.

[0017] Further, the above slave station informs the above master stationof volume of the upstream packet signal to be accumulated in the bufferwhen transmission permission is given to the slave station, and theabove master station controls a value of the timer 2 for permittingtransmission for the above slave station based on upstream packetaccumulation volume informed by the above slave station.

[0018] Further, the above slave station informs the above master stationof ending information indicating that no user IP packet is to betransmitted if no upstream user IP packet is to be transmitted whentransmission permission is given to the slave station, and the abovemaster station receives the ending information from the above slavestation, judges if communication has ended, and allocates transmissionpermission to a next slave station after the above slave station.

[0019] Further, when transmission permission is given to the above slavestation and the above slave station has ended transmission of all of theuser IP packets held, the above slave station informs the above masterstation of ending information indicating that no user IP packet is to betransmitted, and the above master station receives the endinginformation from the above slave station, judges if communication hasended, and allocates transmission permission to a next slave stationafter the above slave station.

[0020] Further, the above master station specifies volume of packetwhich can be transmitted continuously by the above slave station whenthe above master station gives transmission permission to the aboveslave station, and the above slave station ends processing of packettransmission based on the volume of packet which can be transmittedcontinuously, specified for specifying.

[0021] Further, in a packet communication method according to thisinvention, a master station transmits a downstream packet signalincluding identification information for identifying transmissionpermission, and a slave station receives the downstream packet signaltransmitted from the above master station and judges if transmission hasbeen permitted based on an identification number of the downstreampacket signal received.

[0022] Further, a packet communication program according to thisinvention makes a computer execute processing that a master stationtransmits a downstream packet signal including identificationinformation for identifying transmission permission by a master stationand processing that a slave station receives the downstream packetsignal transmitted from the above master station and judges iftransmission has been permitted based on the identification number inthe downstream packet signal received.

[0023] Further, in a computer-readable recording medium according tothis invention, a packet communication program for making a computerexecute processing that a master station transmits a downstream packetsignal including identification information for identifying transmissionpermission and processing that the slave station receives the downstreampacket signal transmitted from the above master station and judges iftransmission has been permitted based on an identification number of thedownstream packet signal received is stored.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 is a functional configuration chart for explaining a PONsystem for an IP packet related to Embodiment 1 and 2.

[0025]FIG. 2 is a configuration chart of a downstream frame signal forexplaining the PON system for the IP packet related to Embodiment 1 and2.

[0026]FIG. 3 is a configuration chart of an upstream packet signal forexplaining the PON system for the IP packet related to Embodiment 1 and2.

[0027]FIG. 4 is a configuration chart of the downstream frame signal forexplaining a downstream frame in FIG. 2.

[0028]FIG. 5 is a configuration chart of the upstream packet signal forexplaining an upstream packet in FIG. 3.

[0029]FIG. 6 is a flow chart concerning allocation algorithm for settingtransmission permission to explain the PON system for the IP packetrelated to Embodiment 1.

[0030]FIG. 7 is a functional configuration chart for explaining the PONsystem for the IP packet, including a function of switching speedrelated to Embodiment 3.

[0031]FIG. 8 is a configuration chart of the downstream frame and theupstream packet signal for explaining the PON system for the IP packet,including the function of switching speed related to Embodiment 3.

[0032]FIG. 9 is a functional configuration chart for explaining the PONsystem for the IP packet, including a function of controlling a timer 2for controlling transmission permission time related to Embodiment 4.

[0033]FIG. 10 is a functional configuration chart for explaining the PONsystem for the IP packet, including the function of controlling thetimer 2 for controlling transmission permission time related toEmbodiment 5.

[0034]FIG. 11 is a functional configuration chart for explaining the PONsystem for the IP packet, including a function of transmitting packetabsence notification related to Embodiment 6.

[0035]FIG. 12 is a system configuration chart in related opticalcommunication.

BEST MODE FOR CARRYING OUT THE INVENTION

[0036] Embodiment 1.

[0037] Embodiment 1 is explained below.

[0038]FIG. 1 is a functional configuration chart of a PON (PASSIVEOPTICAL NETWORK) system for an IP packet for explaining an operation ofthis embodiment. Here, the PON system is a form of an optical subscribernetwork using a passive part such as a star coupler as a component of anetwork. 1 denotes an OLT (master station), 2 denotes an ONU (slavestation), 3 denotes a star coupler for connecting the single OLT 1 and aplurality of ONUs 2 by an optical filer, and 4 denotes the opticalfiber.

[0039] In the OLT 1, 5 denotes an E/O (ELECTRICAL/OPTICAL) circuit forconverting an electrical signal into an optical signal and an O/E(OPTICAL/ELECTRICAL) circuit for converting the optical signal into theelectrical signal. 6 denotes an OAM packet/frame generating circuit forgenerating a downstream frame signal transmitted from the OLT 1 to theONU 2. As illustrated in FIG. 2, this downstream frame signal include'sa downstream OAM (OPERATION AND MAINTENANCE) packet 42 and a user IPpacket area 43. 7 denotes an IP packet terminating circuit forterminating an upstream packet signal from the ONU 2, 8 denotes a signaldetecting circuit for detecting receipt of the upstream packet signalfrom the ONU 2, and 9 denotes a transmission permission sequentialallocation control circuit for permitting an arbitrary ONU 2 to transmitthe upstream packet signal. As stated later, in the PON system for theIP packet in this embodiment, the OLT 1 sequentially allocatestransmission permission of the packet on an upstream transmission lineto the plurality of ONUs 2 registered in advance mainly by using thetransmission permission sequential allocation control circuit.

[0040] Next, in the ONU 2, 10 denotes an O/E-E/O circuit for convertingthe optical signal into the electrical signal and converting theelectrical signal into the optical signal, and 11 denotes an OAMpacket/frame terminating circuit for terminating the downstream framesignal. 12 denotes a transmission permission detecting circuit forcomparing an identifier (ONU_ID) for specifying the ONU 2 set in atransmission permission information 55 area in the downstream OAM packetin the downstream frame signal illustrated in FIG. 4 and an identifier(ONU_ID) held by the ONU itself. The identifier specified in thisdownstream OAM packet has information on to which slave station themaster station is giving transmission permission. 13 denotes an overheadinformation detecting circuit for detecting overhead information 56 inthe downstream OAM packet in the downstream frame signal illustrated inFIG. 4. This overhead information 56 is to inform the ONU 2 of receivingperformance of the OLT 1 itself, and indicates time until the OLT 1becomes able to receive the upstream packet signal stably. Therefore, asillustrated in FIG. 3, the ONU 2 continues to attach a group of signals(overhead 72) which are substantially unnecessary and meaningless on ahead of the upstream packet signal during the time indicated in theoverhead information 56. 15 denotes a packet generating/sending circuitfor generating an upstream packet signal with a header by attaching theoverhead 72 to the upstream packet signal, as stated above. Accordingly,the OLT 1 becomes able to receive the upstream packet signal stably asstated later. 14 denotes a packet-buffer for accumulating an upstream IPpacket input signal 29 input from an outside of the ONU 2.

[0041]FIG. 2 shows a configuration chart for explaining a structure ofthe downstream frame signal transmitted from the OLT 1 to the ONU 2 inthe PON system for the IP packet in this embodiment. In thisexplanation, a frame means one unit (packet) for transmittinginformation. In FIG. 2, 41 denotes a whole downstream frame signal(downstream packet signal) repeated cyclically, 42 denotes thedownstream OAM packet indicating the head of the downstream framesignal, and 43 denotes the user IP packet area. Further, FIG. 3 is aconfiguration chart for explaining a structure of the upstream packetsignal transmitted from the ONU 2 to the OLT 1. In FIG. 3, 44 denotes anexample of configuration of the upstream packet signal. In FIG. 3, oneexample that the overhead 72, the upstream OAM packet 46 (#n) and anupstream user IP packet area 47 (#n) configure a single upstream packetsignal, another example that the overhead 72 and an upstream user IPpacket area 48 (#i) configure a single upstream packet signal, and theother example that the overhead 72 and an upstream OAM packet 49 (#j)configure a single upstream packet signal are illustrated. 46 denotesthe upstream OAM packet (#n) which is one of components of the upstreampacket signal transmitted from the ONU 2 (#n) which is the n-th ONU 2.47 denotes the upstream user IP packet area (#n) which is a component ofthe upstream packet signal transmitted from the ONU 2 (#n) which is then-th ONU 2. 48 denotes the upstream user IP packet area 48 (#i) which isa component of the upstream packet signal transmitted from the ONU 2(#i) which is the i-th ONU 2. 49 denotes the upstream OAM packet 49 (#j)which is a component of the upstream packet signal transmitted from theONU 2 (#j) which is the j-th ONU 2. In FIG. 3, an upstream packet signalarea and a non-signal state are repeated. This is to make the pluralityof ONUs 2 use the transmission line time-divisionally and prevent asingle ONU 2 from occupying the transmission line for a long time bydividing the signal into packets.

[0042]FIG. 4 is a configuration chart showing details of the OAM packetand the user IP packet area illustrated in FIG. 2, configuring thedownstream frame signal in the PON system for the IP packet in thisembodiment.

[0043]51 denotes a downstream packet signal including the downstream OAMpacket and the user IP packet area. The OAM packet of them includes thefollowing components. Specifically, an IP packet header 52, asynchronous pattern 53, packet identification information 54, thetransmission permission information 55 (ONU-ID), the overheadinformation 56, a message area 57, CRC 58 indicating a CRC (CYCLICREDUNDANCY CHECK) operation result for the message area 57, and BIP(BITE INTERLEAVED PARITY) 59 information for applying to a periodbetween a previous OAM packet and a present OAM packet are included.Further, the user IP packet area includes an idle signal 60 interpolatedbetween the packet signals, a user IP packet 1 61, a user IP packet 262, a user IP packet 3 63, etc.

[0044]FIG. 5 shows a configuration chart showing details of the upstreamOAM packet and the upstream user IP packet area illustrated in FIG. 3 inthe PON system for the IP packet in this embodiment. 71 denotes anupstream packet signal including the upstream OAM packet and theupstream user IP packet area. 72 denotes the overhead attached to thehead of the upstream packet signal. The upstream OAM packet configuringthe upstream packet signal includes an IP packet header 73, packetidentification information 74, a message area 75, CRC 76 indicating aCRC operation result for the message area, and BIP 77 for applying to aperiod between a previous OAM packet and a present OAM packet. Further,the upstream user IP packet area includes an idle signal 78 interpolatedbetween the packet signals, a user IP packet 4 79, a user IP packet 580, etc.

[0045]FIG. 6 shows a flow chart concerning allocation algorithm forsetting transmission permission to the plurality of ONUs 2 by the OLT 1in this embodiment.

[0046] S1

[0047] S1 is a step of displaying the ONU identifier (ONU_ID)corresponding to the n-th ONU 2 as identification information in thetransmission permission information 55 area in the downstream OAM packetillustrated in FIG. 4 for OLT 1 to give transmission permission to then-th ONU 2 registered in advance.

[0048] S2

[0049] S2 is a step of starting a timer 1 for detecting packet, which isthe first timer, in consequence of the above step 1 (S1) of displayingthe ONU identifier (ONU_ID) corresponding to the n-th ONU 2 in thetransmission permission information 55 area in the downstream OAMpacket. The timer 1 for detecting packet is provided for judging if theOLT 1 can receive (detect) the upstream packet signal transmitted fromthe n-th ONU 2 to which the OLT 1 has given transmission permissionwithin limit time set by the OLT 1 in the timer 1 for detecting packet.This timer 1 for detecting packet is managed by the OLT 1.

[0050] S3

[0051] S3 is a step of judging if the OLT 1 has received the upstreampacket signal transmitted from the n-th ONU 2 to which the OLT 1 gavetransmission permission. Specifically, the OLT 1 judges if the OLT 1 hasreceived the upstream packet signal transmitted from the n-th ONU 2 towhich the OLT 1 itself gave transmission permission, based on the packetidentification information 74 in the upstream packet signal received,which is illustrated in FIG. 5. The OLT 1 always monitors receipt of theupstream packet signal by receiving the upstream packet signal from then-th ONU 2 within the limit time measured by the timer 1 for detectingpacket or repeating this operation until the limit time measured by thetimer 1 for detecting packet expires.

[0052] S4

[0053] S4 is a step of judging if the timer 1 for detecting packetstarted in step 2 (S2) has expired. As stated above, when the OLT 1 hasnot received the upstream packet signal transmitted from the n-th ONU 2which the OLT 1 has permitted to transmit, in this step, the OLT judgesif the limit time measured by the timer 1 for detecting packet hasexpired. When the limit time has not expired, processing goes back tostep 3 (S3). When the limit time has expired, in step 9 (S9), the ONUidentifier (ONU_ID) is updated for permitting another ONU 2 to transmit(S9). In FIG. 6, one is added to a value of n, however an updatingmethod is not limited to this method. For example, it is also possibleto give transmission permission to the plurality of ONUs 2 uniformly ina focusing or descending order. It is also possible to give priority toa certain ONU 2 in giving transmission permission.

[0054] S5

[0055] S5 is a step of stopping the timer 1 for detecting packet andstarting the timer 2 for controlling transmission permission time(transmission permission timer) which is the second timer, when the OLThas received the upstream packet signal from the n-th ONU 2 to which theOLT 1 gave transmission permission. As illustrated in FIG. 3, the timer2 for controlling transmission permission time is provided for judgingif the OLT 1 can receive all of the upstream packet signals transmittedfrom the n-th ONU 2 within the limit time measured by the timer 2 forcontrolling transmission permission time. This timer 2 for controllingtransmission permission time is managed by the OLT 1.

[0056] S6

[0057] S6 is a step of judging if the limit time measured by the timer 2for controlling transmission permission time started in step 5 (S5) hasexpired. When the limit time measured by the timer 2 for controllingtransmission permission time has expired, in step 9 (S9), the ONUidentifier (ONU_ID) is updated for permitting another ONU 2 to transmit.

[0058] S7

[0059] S7 is a step of judging if the OLT has received all of theupstream packet signals from the n-th ONU 2 to which the OLT 1 gavetransmission permission, when the limit time measured by the timer 2 forcontrolling transmission permission time started in step 5 (S5) has notexpired. When the OLT 1 has not received all of the upstream packetsignals from the n-th ONU 2, the OLT 1 always monitors a state ofreceiving the upstream packet signal by repeating step 6 (S6) of judgingif the limit time measured by the timer 2 for controlling transmissionpermission time has expired and step 7 (S7) of judging if receipt of theupstream packet signal has ended.

[0060] S8

[0061] S8 is a step of stopping the timer 2 for controlling transmissionpermission time when the OLT has received all of the upstream packetsignals transmitted from the n-th ONU 2 to which the OLT 1 gavetransmission permission.

[0062] S9

[0063] As stated above, S9 is a step of updating the identifier (ONU_ID)of the ONU 2 to which the OLT 1 gives transmission permission.

[0064] Next, with reference to FIG. 6, allocation algorithm in each ofthe above steps for setting transmission permission to the plurality ofONUs 2 by the OLT 1 according to this invention is explained.

[0065] The OLT 1 sets the identifier (ONU_ID) for specifying the ONU 2in the transmission permission information 55 area in the downstream OAMpacket for the n-th ONU 2 among the plurality of ONUs 2 registered inadvance (S1). Then, the OLT 1 starts the timer 1 for detecting packet(S2), which is the first timer 1, in consequence of step 1 (S1) forsetting the identifier (ONU_ID) for specifying the ONU 2 in thetransmission permission information 55 area in the downstream OAMpacket. As a timer value in this first timer, time corresponding to atotal value of transmission line loops delay between the OLT 1 and theONU 2 in the PON and processing time in each of circuits of the OLT 1and the ONU 2 can be set arbitrarily. Here, this total value includeslatency time until the OLT 1 actually sends a frame including theidentifier (ONU_ID) as the downstream frame signal, transmission delaytime in the transmission line when the downstream frame signal istransmitted to the ONU 2, processing delay time until the ONU 2 detectstransmission permission directed to the ONU 2 itself after the ONU 2 hasreceived the downstream frame signal, processing delay time until theONU 2 sends the upstream packet signal after the ONU 2 detectstransmission permission, transmission delay time in the transmissionline when the upstream packet signal is transmitted to the OLT 1, andprocessing time necessary for the OLT 1 to detect the upstream packetsignal from the ONU 2 to which the OLT 1 has given transmissionpermission by the signal detecting circuit. The time set in this timer 1for detecting packet can be same time for the plurality of ONUs 2, ordifferent time for each of the ONUs 2.

[0066] The n-th ONU 2 detects the identifier (ONU_ID) in thetransmission permission information 55 area in the downstream OAMpacket, compares the identifier with the identifier held by the n-th ONU2 itself and detects that the identifiers are identical, so that then-th ONU 2 recognizes that transmission permission has been given andsends the upstream packet signal. The OLT 1 judges if the OLT 1 receivesthe upstream packet signal from the n-th ONU 2 to which the OLT 1 hasgiven transmission permission before the first timer expires by usingthe packet identification information 74 in the upstream packet signal(S3). When the OLT 1 detects that this first timer has expired beforethe OLT 1 receives the upstream packet signal from the n-th ONU 2 towhich the OLT 1 has given transmission permission (S4), anidentification number of the ONU 2 to which the OLT 1 gives transmissionpermission is changed from the n-th ONU 2 to the n+1-th ONU 2 (S9). Whenthe OLT 1 has received the upstream packet signal sent by the ONU 2 towhich transmission permission is set before this first timer expires,the OLT 1 continues to give transmission permission, and in consequenceof this, the OLT 1 stops the first timer 1 and at the same time, startsthe second timer for controlling transmission permission time (S5). Thissecond timer for controlling transmission permission time limitstransmission permission time for each ONU 2 in order for assuring eachof ONUs 2 of the upstream band, corresponding to the number of ONUs 2registered in the OLT 1 and service content of the ONUs 2. This timervalue can be set by the OLT 1 arbitrarily. While the OLT 1 is receivingthe upstream packet signal transmitted from the ONU 2 to whichtransmission permission is set, the OLT 1 monitors expiration of thesecond timer for controlling transmission permission time (S6) andexpiration of receipt of the upstream packet signal sent by the ONU 2(S7). When the OLT 1 detects that the limit time measured by the secondtimer has expired before the OLT 1 finishes receiving the packet signalfrom the ONU 2 to which transmission permission is set, the OLT 1 judgesthat maximum time allocated to one ONU 2 is exceeded, and changes theidentification number of the ONU 2 to which the OLT 1 gives transmissionpermission from the n-th ONU 2 to the n+1-th ONU 2 in order to forciblystop setting of transmission permission for the n-th ONU 2 (S9). When itis detected that the OLT 1 has received all of the upstream packetsignals from the ONU 2 to which the OLT 1 gave transmission permission,the second timer is stopped (S8), and the identification number of theONU 2 to which the OLT 1 gives transmission permission is changed fromthe n-th ONU 2 to the n+1-th ONU 2 (S9).

[0067] Further, as stated above, in order for updating the ONUidentifier (ONU_ID) in step 9 (S9) to permit another ONU 2 to transmit,it is not necessarily needed to change the number of the ONU 2 from then-th ONU 2 to the n+1-th ONU 2, and any method can be adopted as far asthe identification number can be updated.

[0068] According to this embodiment, the OLT 1 displays the identifier(ONU_ID) for specifying the ONU 2 to which the OLT 1 gives transmissionpermission in the transmission permission information 55 area in thedownstream OAM packet for the plurality of ONUs 2 registered in advance,and allocates transmission permission of the packet on the upstreamtransmission line to the specific ONU 2 based on this display. Then,with updating of this identifier, the PON system can be configured,which treats the signal for sequentially allocating transmissionpermission of the packet on the upstream transmission line to theplurality of ONUs 2. Therefore, compared to the way that the OLT 1allocates transmission permission to the ONU 2 after the OLT 1 receivestransmission request transmitted from the ONU 2, and controls to avoidcollision of the signals from the ONUs 2 to the OLT 1, as in theabove-stated related technique, the OLT 1 can control allocation oftransmission permission without a step of receiving transmissionpermission. Since the OLT 1 manages an access state of the plurality ofONUs 2 to the upstream transmission line collectively through thismechanism, a controlling procedure of sending and receiving by the OLT 1and ONUs 2 can be simplified. Further, since the OLT 1 manages theaccess state of the plurality of ONU's to the upstream transmission linecollectively, advantages of using the transmission line effectively andperforming packet communication efficiently can be obtained.

[0069] Further, since the OLT 1 applies control algorithm forcontrolling two kinds of timers independently, advantages such aspreventing a single ONU 2 from occupying transmission and providingopportunity of transmission of the packet for the plurality of ONUs 2can be obtained. Embodiment 2.

[0070] Embodiment 2 is explained below.

[0071] This embodiment of the invention is characterized in that theupstream packet signal is reproduced accurately by using an area fordisplaying the overhead information 56 in the downstream OAM packet in adownstream packet signal 51 described in FIG. 4 and an overhead 72 areaon the head of the upstream packet signal 71 described in FIG. 5.

[0072] Firstly, operations concerning the overhead information 56 in theOAM packet in the downstream packet signal in FIG. 4 and the overhead 72attached to the head of the upstream packet signal in FIG. 5 areexplained.

[0073] The optical receiving unit (particularly representing the O/E forconverting the optical signal into the electrical signal in O/E-E/O inFIG. 1) in the OLT 1 firstly adjusts a gain of a receiver based on adifference in strength of the optical signal caused by a difference in alength of the optical fiber 4 from the OLT 1 to each of the ONUs 2, whenthe optical receiving unit receives the upstream packet signal from theONU 2. After the gain becomes stable, the upstream packet signal isreproduced as the electrical signal. Here, the gain is a ratio of outputto input of the optical receiver. In this case, a head part of theupstream packet signal is not reproduced or becomes an indefinite signalduring a period until the gain of the optical receiver is stable.Therefore, the overhead 72 corresponding to the period until the gain ofthe optical receiver is stable is set on the head of the upstream packetsignal. This overhead 72 is a group of signals of meaninglessinformation.

[0074] The OLT 1 sets arbitrary header length information (overheadinformation 56) which is to be attached to the head of the upstreampacket signal, in the downstream OAM packet, so that the OLT 1 enablesthe ONU 2 to send the upstream packet signal including arbitrary headerlength (overhead 72) corresponding to the overhead information 56 in anupstream transmission direction.

[0075] Therefore, it is not necessary that the packet analyzing unitanalyzes the packet length signal described in the packet overhead (LLCpart) in the first packet signal from the subscriber's equipment as inthe related technique. Since the ONU 2 attaches the overhead 72 to theupstream packet signal, the OLT 1 reproduces a signal which is notrequired to be reproduced and stored in the overhead 72 area, until thegain of the optical receiver becomes stable. Then, the OLT 1 accuratelyreproduces the necessary upstream packets after the gain of the opticalreceiver becomes stable, therefore, it becomes possible to accuratelyreproduce the upstream packet signal transmitted from the ONU 2.Further, there is a case that receiving performance of the opticalreceiver in the OLT 1 differs in each OLT 1. Therefore, by displayingthe overhead information 56 appropriate for the performance of theoptical receiver in the OLT 1 in a downstream OAM packet signalarbitrarily, it becomes possible to send the upstream packet signal withan arbitrary header length (overhead 72) in the upstream transmissiondirection. Therefore, since length of the overhead 72 can be changedflexibly based on the receiving performance of the optical receiver ofthe OLT 1, there is an advantage of reproducing the upstream packetsignal accurately. Embodiment 3.

[0076] Embodiment 3 is explained below.

[0077]FIG. 7 is a functional configuration chart in the PON system forexplaining an operation of a function of switching speed in thisembodiment.

[0078] In the OLT 1, 84 denotes a buffer for various speeds foraccumulating the IP packet signals (downstream packet signals) frominterfaces for various speeds, separately according to signaltransmission speeds, 85 denotes an OAM packet/frame generating circuitfor generating the downstream frame signal including the OAM packetseparately according to speeds, 5 denotes the O/E-E/O circuit forconverting the electrical signal into the optical signal and the opticalsignal into the electrical signal, 87 denotes an IP packet terminatingcircuit for variable speed for terminating the upstream signal from theONU 2 separately according to speeds, 8 denotes the signal detectingcircuit for detecting receipt of the packet signal from the ONU 2, and89 denotes a transmission permission sequential allocation controlcircuit for permitting the ONU 2 with the specific identifier totransmit the upstream packet signal by generating the downstream frame(downstream packet signal) for the ONUs 2 of various speedstime-divisionally and setting the identifier (ONU_ID) for specifying theONU 2 in the transmission permission information 55 in the downstreamOAM packet.

[0079]FIG. 8 is a configuration chart, for explaining this embodiment,of the downstream signal and the upstream signal in the PON system forthe IP packet including the function of switching speed. Theconfiguration chart of a downstream frame signal 41 illustrated in FIG.8 shows an example of frame signal that is used when a plurality of theONU 2 for 1 Gbps (BITE PER SECOND), the ONU 2 for 10 Gbps and the ONU 2for 100 Mbps is connected to the same OLT 1. The upstream packet signal44 illustrated in FIG. 8 is a configuration chart of the upstream packetsignal corresponding to the downstream packet signal set separatelyaccording to signal transmission speeds among the downstream packetsignals 41. Here, bps indicates a transmission speed in Ethernet whenthe Ethernet is provided for an UNI (USER NETWORK INTERFACE).

[0080] Next, with reference to FIG. 7 and FIG. 8, the operation of thefunction of switching speed is explained.

[0081] The OLT 1 reads out the IP packet signal of a specific signaltransmission speed from the buffer 84 for various speeds, whichaccumulates the downstream packet signal separately according to signaltransmission speeds. Then, the OLT 1 sets the identifier (ONU_ID) forspecifying the ONU 2 in the transmission permission information 55 areain the OAM packet in the downstream packet signal, in order to givetransmission permission to the ONU 2 of an arbitrary speed among theplurality of ONUs 2 registered separately according to speeds. Afterthen, the OLT 1 transmits the downstream packet signal including the IPpacket signal and the OAM packet signal which has the overheadinformation 56 to the ONU 2. In this case, only the ONU 2 which has beensynchronized with the transmission speed of the OLT 1 can receive thisdownstream packet signal, and implement OAM packet termination process.The ONU 2 which has established synchronization with the downstreampacket signal and implemented termination process for the OAM packetsignal, detects the identifier (ONU_ID) in the transmission permissioninformation 55 area in the OAM packet. After confirming identity, theONU 2 transmits the upstream packet signal to the OLT 1. The aboveoperation is repeated for each signal transmission speed. Theconfiguration of the ONU 2 is identical with the configuration of theONU 2 explained in embodiment 1. However, in this embodiment, the ONUs 2of various signal transmission speeds can be mixed in the system.Further, when a plurality of ONUs 2 synchronized with the transmissionspeed of the OLT 1 is mixed in the system, it is possible to apply theallocation algorithm explained in embodiment 1, which is for settingtransmission permission by the OLT 1 to the plurality of ONUs 2.

[0082] As stated, since the OLT 1 includes the buffer 84 for variousspeeds for accumulating the downstream packet signal separatelyaccording to signal transmission speeds, this embodiment has advantagesthat the configuration of the PON system in which the plurality of ONUs2 of various signal transmission speeds is mixed is realized and that anoperation, such as relocation of the network due to increase intransmission volume of the ONU 2 becomes unnecessary.

[0083] Embodiment 4.

[0084] Embodiment 4 is explained below.

[0085]FIG. 9 is a functional configuration chart of the PON system forexplaining the function of controlling the timer 2 for controllingtransmission permission time.

[0086] In the OLT 1, 110 denotes a packet volume monitor-timer 2 controlunit which monitors packet communication volume in each of the ONUs 2 byreceiving a packet detecting signal 27 and determines a value of thetimer 2 for controlling transmission permission time dynamically, and111 denotes the timer 2 control signal which notifies the transmissionpermission sequential allocation control circuit 9 of a value of thetimer 2 for controlling transmission permission time.

[0087] Next, with reference to FIG. 9, a control operation on the timer2 for controlling transmission permission time is explained.

[0088] The packet volume monitor-timer 2 control unit 110 monitorsupstream packet communication volume of each of the ONUs 2. When theupstream packet communication volume is large, the packet volumemonitor-timer 2 control unit 110 notifies the transmission permissionsequential allocation control circuit 9 by the timer 2 control signal111 to increase the value of the timer 2 for controlling transmissionpermission time which determines time for giving transmission permissionto the relevant ONU 2. When the upstream packet communication volume issmall, the packet volume monitor-timer 2 control unit 110 notifies thetransmission permission sequential allocation control circuit 9 by thetimer 2 control signal 111 to decrease the value of the timer 2 forcontrolling transmission permission time which determines time forgiving transmission permission to the relevant ONU 2. As a method forjudging if the upstream communication volume of each of the ONUs 2 islarge or small by the packet volume monitor-timer 2 control unit 110, itis possible to judge based on the upstream packet communication volumein a certain period of time. It is also possible to judge based on theupstream packet communication volume in a period between time whentransmission permission is given to the relevant ONU 2 at once and timewhen transmission permission is given to a next ONU 2.

[0089] As stated, the OLT 1 includes the packet volume monitor-timer 2control unit 110 which monitors the upstream packet communication volumeof each of the ONUs 2 and determines the value of the timer 2 forcontrolling transmission permission time for determining time for givingtransmission permission to the relevant ONU 2 based on the upstreampacket communication volume. Therefore, in this embodiment, it becomespossible to increase time for giving transmission permission to the ONU2 which has large upstream packet communication volume and decrease timefor giving transmission permission to the ONU 2 which has small upstreampacket communication volume, and even when volume of generated upstreampacket to be communicated differs in the ONUs, there is an advantage ofcommunicating efficiently.

[0090] Embodiment 5.

[0091] Embodiment 5 is explained below.

[0092]FIG. 10 shows a functional configuration chart of the PON systemfor explaining the function of controlling the timer 2 for controllingtransmission permission time in this embodiment.

[0093] In the ONU 2, 120 denotes a packet buffer monitor circuit formonitoring packet accumulation volume in the packet buffer 14 andsetting the packet accumulation volume in the upstream OAM packet, 121denotes the packet buffer monitor signal for notifying the above packetbuffer monitor circuit 120 of the packet accumulation volume, 122denotes the packet accumulation information signal for setting thepacket accumulation volume in the packet buffer 14 in the upstream OAMpacket, 130 denotes an ONU packet volume monitor-timer 2 control unitfor receiving the packet accumulation information in the ONU anddetermining the value of the timer 2 for controlling transmissionpermission time for the relevant ONU dynamically, 131 denotes the ONUpacket accumulation information for notifying the ONU packet volumemonitor-timer 2 control unit 130 of the packet accumulation informationset in the upstream OAM packet, and 132 denotes the timer 2 controlsignal for notifying the transmission permission sequential allocationcontrol circuit 9 of the value of the timer 2 for controllingtransmission permission time.

[0094] Next, with reference to FIG. 10, the control operation on thetimer 2 for controlling transmission permission time is explained.

[0095] The packet-buffer monitoring circuit 120 in the ONU 2 monitorsthe upstream packet volume accumulated in the packet-buffer 14, and whentransmission permission is received from the OLT 1, the packet-buffermonitoring circuit 120 sets the upstream packet volume as the packetaccumulation information in the message area 75 in the upstream OAMpacket 71 and sends the upstream OAM packet to the OLT 1.

[0096] The ONU packet volume monitor-timer 2 control unit 130 in the OLT1 receives the ONU packet accumulation information 131 set in themessage-area 75 as the packet accumulation information in the aboveupstream OAM packet 71, and when the packet accumulation volume in therelevant ONU 2 is large, the ONU packet volume monitor-timer 2 controlunit 130 notifies the transmission permission sequential allocationcontrol circuit 9 to increase the value of the timer 2 for controllingtransmission permission time which determines time for givingtransmission permission to the relevant ONU 2 by using the timer 2control signal 111. When the upstream packet accumulation volume issmall, the ONU packet volume monitor-timer 2 control unit 130 notifiesthe transmission permission sequential allocation control circuit 9 todecrease the value of the timer 2 for controlling transmissionpermission time which determines time for giving transmission permissionto the relevant ONU 2 by using the timer 2 control signal 111.

[0097] As stated, since the ONU 2 to which the OLT 1 has giventransmission permission has the packet buffer monitoring circuit formonitoring the upstream packet accumulation volume in the packet bufferand notifying the OLT 1 of the accumulated packet volume when therelevant ONU 2 receives transmission permission, and the OLT 1 has theONU packet volume monitor-timer 2 control unit 130 for receiving thepacket accumulation information in the above ONU 2 and determining thevalue of the timer 2 for controlling transmission permission time whichdetermines time for giving transmission permission to the relevant ONU 2based on the accumulation volume, in this embodiment, it become possibleto increase time for giving transmission permission to the ONU 2 whichhas large upstream packet accumulation volume and decrease time forgiving transmission permission to the ONU 2 which has small upstreampacket accumulation volume. Therefore, there is an advantage ofcommunicating efficiently even when volume of the generated upstreampacket to be communicated differs in the ONUs

[0098] Embodiment 6.

[0099] Embodiment 6 is explained below.

[0100]FIG. 11 is a flow chart concerning allocation algorithm forsetting transmission permission to the plurality of ONUs 2 by the OLT 1in this embodiment.

[0101] In FIG. 11, step 10 (S10) stated below is added to the flow chartillustrated in FIG. 6. Since steps other than step 10 (S10) are statedearlier, explanation is omitted.

[0102] S10

[0103] S10 is a step of judging if the OLT 1 has received endinginformation indicating that transmission of the upstream user IP packetfrom the ONU 2 has ended. When the OLT 1 has received the endinginformation from the ONU 2, processing goes to step 8 (S8) and the OLT 1stops the timer 2 for controlling transmission permission time. When theOLT 1 has not received the ending information from the ONU 2, processinggoes back to step 6 (S6), and the OLT 1 judges if the timer 2 forcontrolling transmission permission time has expired.

[0104] Next, with reference to FIG. 11, packet receipt ending judgmentcontrol based on receipt of the ending information from the ONU 2 isexplained.

[0105] If the ONU 2 in this embodiment does not have the upstream userIP packet to be transmitted when the OLT 1 has given transmissionpermission to the ONU 2, the ONU 2 notifies the OLT 1 of the endinginformation in the message area 75 in the upstream OAM packet 49, whichinforms that there is no upstream user IP packet. Further, while the OLT1 is giving transmission permission and the upstream user IP packet isbeing transmitted, when there is no upstream user IP packet to betransmitted next, the ONU 2 notifies the OLT 1 of the ending informationin the message area 75 in the upstream OAM packet 49, which informs thatthere is no upstream user IP packet.

[0106] In the allocation algorithm for setting transmission permissionto the plurality of ONUs 2 in FIG. 11, the OLT 1 judges if the OLT 1 hasreceived the ending information from the ONU 2 in step 10 (S10). If theOLT 1 has received the ending information, the OLT 1 stops the timer 2for controlling transmission permission time in step 8 (S8), and updatesthe identifier (ONU_ID) of the ONU 2 to which the OLT 1 givestransmission permission from step 9 (S9) on. When the OLT 1 has notreceived the ending information from the ONU 2, the processing goes backto step 6 (S6). Then, the OLT judges if the timer 2 for controllingtransmission permission time has expired and continues to receive theupstream user IP packet 79.

[0107] As stated, the ONU 2 includes a function of informing the OLT 1by using the message area 75 in the upstream OAM packet as the endinginformation when there is no upstream user IP packet to be sent, and theOLT includes a function of judging if upstream user IP packettransmission has ended by receiving the ending information from the ONU2. Therefore, it becomes possible to give transmission permission to anext ONU 2 without waiting until the timer 2 for controllingtransmission permission time expires. Hence, there is an advantage ofreducing useless latency time and allocating transmission permissionefficiently.

[0108] Embodiment 7.

[0109] Embodiment 7 is explained below.

[0110] The OLT 1 in this embodiment specifies volume (value of time ordata volume) of the packet which an ONU 2 can send continuously in themessage area 57 in the downstream OAM packet when the OLT 1 givestransmission permission to the ONU 2. The ONU 2 receives transmissionpermission and the specified volume of the packet which can be sentcontinuously, and the ONU 2 stops an operation of sending the packetwhen packet transmission in a specified range has ended even if there isstill the upstream packet to be sent.

[0111] As stated, the OLT 1 includes a function of specifying the volumeof the packet which the ONU 2 can send continuously and determining thevalue of the timer 2 for controlling transmission permission time basedon this specified volume when the OLT 1 gives transmission permission tothe ONU 2, and the ONU 2 includes a function of stopping the operationof sending the packet when transmission of the packet in the specifiedrange ends in a case that the OLT 1 has given transmission permissionand specified the volume of the packet which can be sent continuously.Accordingly, the ONU 2 can stop transmission of the packet without beingforced to stop by the expiration of the timer 2 for controllingtransmission permission time, and there is an advantage of allocatingtransmission right to the plurality of ONUs efficiently.

[0112] So far, in all of the embodiments, explanation was made on thePON system. However, it is not necessary to limit this invention to thePON system, and this invention can be used in a transmission method forpacket communication. Therefore, concerning the transmission line, it isnot necessary to limit the transmission line to the optical fiber, andas far as packet communication can be performed, an exclusive line, etc.can be used.

[0113] Further, in all of the embodiments, an embodiment of sending andreceiving between the master station (OLT) and the slave station (ONU)has been illustrated. However, this system is not limited to thecommunication system including the master station and the slave station,and this system can be applied to the communication system in generalincluding a transmitter and a receiver.

[0114] Terms of “accumulate” and “display” which are described so farmean storing in a recoding medium.

[0115] Further, in all of the embodiments, each of operations in each ofelements are mutually related, and the operation in each of the elementscan be replaced with a series of operations in considering relationshipamong the operations illustrated above. By replacing in this way, it ispossible to realize embodiments of invention of the method. Further, byreplacing the operation in each of the above elements with processing ineach of the elements, it is possible to realize embodiments of a programand embodiments of a computer-readable recording medium in which theprogram is stored. These embodiments can be configured by a programwhich can be fully operated in the computer. Each of processing in theembodiments of the program and the embodiments of the computer-readablerecording medium in which the program is stored is executed by theprogram. However, this program is stored in a recording unit, and readfrom the recording unit to a central processing unit (CPU), and each ofthe flow charts are executed by the central processing unit. Therecording unit and the central processing unit are not illustrated.

[0116] Further, software and the program in each of the embodiments canbe also realized as firmware stored in ROM (READ ONLY MEMORY). Or, it isalso possible to realize each of functions in the aforementioned programby combining the software, the firmware and hardware.

INDUSTRIAL APPLICABILITY

[0117] According to this invention, since the OLT allocates transmissionpermission of the packet on the upstream transmission line to theplurality of ONUs registered without receiving transmission request, itis possible to simplify sending and receiving.

[0118] Further, the ONU can transmit the upstream packet signal based ontransmission permission information of the OLT.

[0119] Further, it is possible to limit time of receiving the upstreampacket signal transmitted from the ONU to which transmission permissionhas been given, by timer control of the timer 1 for detecting packet.

[0120] Further, it is possible to limit time of ending receipt of theupstream packet signal transmitted from the ONU to which transmissionpermission has been given, by timer control of the timer 2 forpermitting transmission

[0121] Further, it becomes possible to reproduce the upstream packetsignal accurately by setting the overhead in the length appropriate forthe performance of the optical receiver of the OLT.

[0122] Further, it is possible to mix the plurality of ONUs of varioussignal transmission speeds in a same system by providing a buffer forvarious signal transmission speeds in the OLT.

[0123] Further, it is possible to change the value of the timer 2 forcontrolling transmission permission time by providing a function ofmonitoring packet volume in the OLT, and even when the volume of thegenerated upstream packet to be communicated differs in the ONUs, it ispossible to communicate efficiently.

[0124] Further, by providing the packet accumulation volume monitor forthe packet buffer and a function of notifying the OLT in the ONT and byproviding a function of receiving the packet accumulation information inthe OLT, it is possible to change the value of the timer 2 forcontrolling transmission permission time. Even when the generated volumeof the upstream packet to be communicated differs in the ONUs, it ispossible to communicate efficiently.

[0125] Further, since the function of notifying the OLT of the endinginformation of transmission is provided in the ONU and a function ofpermitting a next ONU to transmit by receiving the ending information isprovided in the OLT, it becomes possible to give transmission permissionto the next ONU without waiting until the timer 2 for controllingtransmission permission time expires. It is possible to allocate thetransmission right efficiently.

[0126] Further, since a function of specifying the volume of the packetwhich can be transmitted continuously when the OLT gives transmissionpermission to the ONU is provided in the OLT and a function of stoppingthe operation of sending the packet when transmission reaches the volumeof the packet which can be transmitted continuously specified at time ofpermitting transmission is provided in the ONU, the ONU can stoptransmitting without being forced to stop by the expiration of the timer2 for controlling transmission permission time. The OLT can givetransmission permission to the next ONU, and the transmission right canbe allocated efficiently among the plurality of ONUs.

[0127] Further, by a method of sequentially allocating transmissionpermission of the packet on the upstream transmission line from the OLTto the plurality of ONUs registered, there is an advantage ofsimplifying sending and receiving in the packet communication.

[0128] Further, by the program for sequentially allocating transmissionpermission of the packet on the upstream transmission line from the OLTto the plurality of ONUs registered, processing of packet communicationin a simplified sending and receiving method can be executed on thecomputer.

[0129] Further, the OLT uses a computer-readable recording medium whichrecords the program for sequentially allocating transmission permissionof the packet on the upstream transmission line to the plurality of ONUsregistered as a medium, and the OLT can execute the above processing ofpacket communication on the computer by the program read by the computerfrom the above recording medium.

1. A packet communication system comprising: a master station fortransmitting a downstream packet signal including identificationinformation for identifying transmission permission in order to givetransmission permission without receiving transmission request to themaster station itself; and a slave station for receiving the downstreampacket signal transmitted from the above master station and judging iftransmission has been permitted based on the identification informationin the downstream packet signal received.
 2. The packet communicationsystem of claim 1, wherein the above slave station transmits an upstreampacket signal to the master station when the slave station judges thattransmission has been permitted.
 3. The packet communication system ofclaim 2, wherein the above master station includes a timer 1 fordetecting packet which measures limit time when the master station canjudge if the upstream packet signal has been transmitted from the aboveslave station to which transmission has been permitted, wherein themaster station judges if the master station has received the upstreampacket signal transmitted from the above slave station within the limittime measured by the timer 1 for detecting packet, stops the timer 1 fordetecting packet when the master station has received the upstreampacket signal, and updates the identification information foridentifying transmission permission when the master station has notreceived the upstream packet signal.
 4. The packet communication systemof claim 3, wherein the above master station includes a timer 2 forpermitting transmission which measures limit time when the masterstation can continuously receive the upstream packet signal transmittedfrom the above slave station to which transmission has been permitted,wherein the master station judges if receipt of the upstream packetsignal transmitted from the above slave station has ended within thelimit time measured by the timer 2 for permitting transmission, stopsthe timer 2 for permitting transmission when the receipt has ended, andupdates the identification information for identifying transmissionpermission when the receipt has not ended.
 5. The packet communicationsystem of claim 2, wherein the above master station transmits adownstream packet signal including overhead information indicating timewhen output of the upstream packet signal is unstable, wherein the aboveslave station adds an overhead which is a group of signals which are notreproduced by the above master station on a head of the upstream packetsignal corresponding to the overhead information in the downstreampacket signal transmitted from the above master station.
 6. The packetcommunication system of claim 1, wherein the above master stationincludes a buffer for accumulating the downstream packet signalseparately according to signal transmission speeds, wherein the masterstation reads out the downstream packet signal of various speedsaccumulated in the buffer separately according to the signaltransmission speeds, transmits the downstream packet signal of thevarious speeds read out, and permits a slave station which has been ableto be synchronized with transmission speed of the downstream packetsignal to transmit an upstream packet signal.
 7. The packetcommunication system of claim 4, wherein the above master stationmonitors upstream packet communication volume of each slave station andcontrols a value of the timer 2 for permitting transmission for theabove slave station based on the upstream packet communication volume.8. The packet communication system of claim 4, wherein the above slavestation informs the above master station of volume of the upstreampacket signal to be accumulated in the buffer when transmissionpermission is given to the slave station, wherein the above masterstation controls a value of the timer 2 for permitting transmission forthe above slave station based on upstream packet accumulation volumeinformed by the above slave station.
 9. The packet communication systemof claim 4, wherein the above slave station informs the above masterstation of ending information indicating that no upstream packet signalis to be transmitted when no upstream packet signal is accumulated inthe buffer when transmission permission is given to the slave station,wherein the above master station receives the above ending informationinformed by the above slave station and gives transmission permission toa next slave station after the above slave station without waiting untilthe timer 2 for permitting transmission expires.
 10. The packetcommunication system of claim 4, wherein when transmission permission isgiven to the above slave station and the above slave station has endedtransmission of all of the upstream packet signals accumulated in thebuffer to the above master station, the above slave station informs theabove master station of ending information indicating that there is noupstream packet signal is to be transmitted, wherein the above masterstation receives the above ending information informed by the aboveslave station and gives transmission permission to a next slave stationafter the above slave station without waiting until the timer 2 forpermitting transmission expires.
 11. The packet communication system ofclaim 4, wherein the above master station specifies volume of packetwhich can be transmitted continuously by the above slave station whenthe above master station gives transmission permission to the aboveslave station, wherein the above slave station ends processing of packettransmission based on the volume of packet which can be transmittedcontinuously, specified for specifying.
 12. A packet communicationmethod, wherein a master station transmits a downstream packet signalincluding identification information for identifying transmissionpermission, wherein a slave station receives the downstream packetsignal transmitted from the above master station and judges iftransmission has been permitted based on an identification number of thedownstream packet signal received.
 13. A packet communication programcharacterized in processing that a master station transmits a downstreampacket signal including identification information for identifyingtransmission permission by a master station and processing that a slavestation receives the downstream packet signal transmitted from the abovemaster station and judges if transmission has been permitted based onthe identification number in the downstream packet signal received. 14.A computer-readable recording medium for recording a packetcommunication program for making a computer execute processing that amaster station transmits a downstream packet signal includingidentification information for identifying transmission permission andprocessing that the slave station receives the downstream packet signaltransmitted from the above master station and judges if transmission hasbeen permitted based on an identification number of the downstreampacket signal received.